Polymers and polymerization methods provide important materials such as specialty plastics, foams, gels, and rubbery materials. Despite the many advances in the past decades realized through polymer chemistry, significant imperfections and challenges remain, for example, the aging and cracking of the materials and leaching of additives. (Stevens, M. P. 1990 in Polymer Chemistry: an Introduction, Oxford University Press, New York; Odian, G. 2004 in Principles of Polymerization, Wiely, New York; Murphy, J. 2001 in Additives for Plastics Handbook, Elsevier, New York.)
Environmental contamination and bioaccumulation of additives, such as flame-retardants, plasticizers, and anti-oxidants, are particularly problematic. Added inorganic salts compromise the physical/mechanical properties of polymers. Halogenated flame retardant additives are bio-accumulative, and thus threaten the environment and human health. (Blum, et al. 1997 Science 195, 17-23; Gold, et al. 1978 Science 200, 785-787; Fishbein, 1985 Carcinog. Mutagens Envron. 5, 75-93; de Boer, et al. 1998 Nature 394, 28-29; Hale, et al. 2001 Nature 412, 140-141; Rahman, et al. 2004 Prog. Polym. Sci. 29, 1223-1248; g) Gomara, et al. 2007 Environ. Sci. Technol. 41, 6961-6968; Blum 2007 Science 18, 194; Lyons, 1970 in Chemistry and Uses of Fire Retardants, Wiely, New York; Camino, et al. 2001 Polym. Degrad. Stab. 74, 457-464.)
Due to their excellent adhesive properties, processability and low cost, epoxy and cyanate ester resins are used prominently as polymer matrix composites. (Pilato, et al. 1994 Advanced composite materials; Springer, Berlin Heidelberg New York; Lee, et al. 1967 Handbook of epoxy resins; McGraw Hill, New York; McAdams, et al. 1988 Encyclopedia of polymer science and engineering; Wiley, New York, Vol. 6, p 322-382; Nair, et al. 2001 Adv. Polym. Sci. 155, 1-99.) For example, bisphenol-A (BPA) epoxy formulations dominate the epoxy resin market, but generally contain fillers, curing agents, and flame retardants (FRs). (Pham, et al. 2004 Kirk-Othmer encyclopedia of chemical technology; Wiley, New York, Vol. 10, p 347-461.) The inherent flammability of such materials is particularly problematic in settings that require flame resistance, such as vehicles, and electronic and construction materials. Brominated FR small molecules such as polybrominated diphenyl ethers, hexabromocyclododecane, and tetrabromobisphenol A (TBBPA) have been implemented widely to reduce flammability. (Levchik, et al. 2004 Polym. Int. 53, 1901-1929; Guerra, et al. 2011 The Handbook of Environmental Chemistry: Introduction to brominated flame retardants: commercially products, applications, and physicochemical properties; Springer, Berlin Heidelberg.) However, many brominated FRs were recently classed together with halogenated FRs, and present serious health and environmental concerns, such as bioaccumulation and associated carcinogenicity. Potential negative effects of such molecules on the environment, and in animals and humans, remain controversial and are under continued investigation. (Communication from the commission on the risk evaluation and the risk reduction strategies for the substances, C152, Official Journal of the European Union, 2008; Debenst, et al. 2010 J. Environ. Monit. 12, 1918-1923; McCormick, et al. 2010 Aquatic Toxicology 100, 255-262; de Wit, et al. 2011 The Handbook of Environmental Chemistry: Emerging brominated flame retardants in the environment; Springer, Berlin Heidelberg.)
Cyanate ester (CE) resins can cure thermally by cyclotrimerization to give polycyanurates, which exhibit high glass transition temperatures, toughness and low dielectric properties, making them useful in high performance structural and electronics applications. (Hamerton, I. (ed) 1994 Chemistry and technology of cyanate ester resins; Blackie Academic and Professional, Glasgow, UK.) Halogen-free CE resins (e.g., BPA, bisphenol-E, and bisphenol-M CE resin) exhibit undesirably high flammability, with a heat release capacity (HRC) of 240-320 J/(g-K), total heat release (THR) of 15-23 kJ/g, and char yield of 26-42%. (Lyon, et al. 2006 Fire Mater. 30, 89-106.) Moreover, the long gelation time of CE resins is frequently in need of adjustment through the addition of curing catalysts, such as imidazoles or transition metal complexes. (Fang, et al. 1995 Progr. Polym. Sci. 20, 61-118; Liu, et al. 1996 Polymer 37, 3675-3682.)
Thus, methods and materials that address the above problems are urgently needed. Novel polymer and synthetic methods that provide non-flammable properties in the absence of additives will benefit society in terms of both performance and safety.